Mighty Tevatron Shuts Down After Decades of Discoveries

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Mighty Tevatron Shuts Down After Decades of Discoveries

The Tevatron was the world's most powerful particle accelerator and the site of many of the biggest discoveries in the field for more than two decades. It symbolized the United States' scientific dominance and was the center of the international physics world until it was eclipsed by Europe's Large Hadron Collider last year.
Today, protons and antiprotons will stop speeding around the 4-mile-long circular accelerator track at Fermilab in Batavia, Illinois, and data from the Tevatron's final particle collisions will be recorded. The end will be marked by a ceremony to turn the machine off and celebrate the dozens of discoveries it made, broadcast live online at 2 p.m. central time.
“For the last 28 years, the Tevatron has been the real workhorse of particle physics,” said physicist Paul Halpern of the University of the Sciences in Philadelphia and author of the book Collider. Without it, physicists would have a large gap in their knowledge of the universe, he added.
Over the course of its life, the Tevatron has made a number of remarkable findings related to the Standard Model of physics, which describes the characteristics and behavior of subatomic particles. The facility's two detectors, the Collider Detector at Fermilab (CDF) and DZero experiments, have both competed and worked together to reveal the nature of matter.
Even after Europe's Large Hadron Collider began producing higher-energy beams, and budget constraints forced the lab to schedule its shutdown, the Tevatron continued to perform and put in an impressive ninth-inning effort in pursuit of the still-elusive Higgs boson, the last undiscovered particle predicted by the Standard Model. And there is still a chance it could succeed, as scientists continue to analyze the Tevatron's data, even after the machine goes dark.
Here are some of the Tevatron's greatest achievements.
Above:

Ring of Energy

The Tevatron accelerates charged particles to extremely high speeds using a four-mile long ring of magnets. The instrument then slams the particles together in order to split them up into their constituents. The Tevatron typically shoots a beam of protons and antiprotons in opposite directions around the ring until they reach a desired energy range and then allows the two beams to meet. The ensuing collision produces a rain of particles that researchers then dig through to search for those predicted by the Standard Model.
The accelerator produces an extremely bright beam of these particles and is able to reach energies up to 1 TeV, or roughly a trillion times the energy of ordinary visible light. The facility was the world-record holder in high-energy physics until it was surpassed by the LHC, which will ultimately be able to reach energies seven times those of the Tevatron’s peak.
Image: Fermilab

Top Quark

Arguably the Tevatron’s most important discovery was that of the top quark in 1995. Quarks — the fundamental building blocks of protons and neutrons — are one of the hallmarks of the Standard Model of modern physics. These particles come in six “flavors": up, down, strange, charmed, top and bottom.
The top quark, being the most massive of the quarks, was the last to be discovered. When spotted by CDF and DZero, the top quark’s observed mass was extremely close to what had been predicted under the Standard Model, providing a nice verification of the theory.
Image: Reidar Hahn/Fermilab

The Last Meson

Particles such as protons and neutrons are called baryons and are combinations of three quarks. But two quarks can also combine into particles known as mesons, which are made of a quark and an anti-quark. There are 15 allowable combinations that produce mesons and, until 1998, all had been spotted in nature or other experiments. It took the power of the Tevatron to uncover the BC (B sub see) meson, which was the heaviest.
Image: CDF detector; Reidar Hahn/Fermilab

Violation Observation

Most subatomic particles last only a few slivers of a second after a collision and then decay into other particles. These decays obey certain symmetries, such as the fact that they occur the same way regardless of the charge or direction of the interacting particles.
But after observing neutral kaons, researchers found the particles exhibiting a slight difference when the decay occurred in one direction versus the mirror-flipped direction. This result provided some justification for an otherwise odd paradox in the universe: why it is filled with matter instead of anti-matter.
Since both matter and anti-matter are thought to be equivalent in every way but charge, physicists had long thought they should be produced in roughly equal proportions after the Big Bang. But when a particle meets its anti-particle, both are instantly destroyed. So how could it be that the universe is filled with matter while anti-matter remains very rare?
The observed symmetry violation, known as charge-parity or CP violation, was slight yet large enough to help account for the dominance of matter — which makes up everything in the known universe — over anti-matter.
*Image: *Reidar Hahn/Fermilab

Tau Neutrino

According to the Standard Model, neutrinos come in three types: electron, muon, and tau. Using the Tevatron, researchers with the Direct Observation of the Nu Tau (DONUT) experiment spotted the heaviest of these three, the tau neutrino, in 2000. While indirect evidence had previously been established for its existence, this was the first direct confirmation of the particle.
Neutrinos are ghostly particles so small they pass through ordinary matter as easily as a bullet through a fog bank. It took researchers three years of observation and sifting through six million potential interactions to tease out the tau neutrino.
Image: Fermilab

Cascading Bosons

In 2007, scientists produced the equivalent of a subatomic particle Frankenstein. As it turns out, the six quark flavors are separated into three pairs called “generations” that correspond to their energy levels. Only particles containing specific combinations of these generations had been seen in other experiments.
Then the Tevatron produced a particle known as a cascading B baryon that was composed of a bottom quark, a down quark, and strange quark — a member from each of the three quark generations. The finding was the first time this combination of generations had been observed.
“The discovery opened up the scope of the particle zoo and showed us that what’s out there is more exotic than once believed,” said physicist Paul Halpern.
Image: Fermilab

Weak Force

Over its lifetime, the Tevatron managed to pin down several particle masses. In 2009, the DZero experiment announced the most precise measurement of the W boson, one of the carriers of the weak force.
The weak force is responsible for many subatomic and radioactive processes and is mediated by the W and Z bosons, much the same way that the electromagnetic force is mediated by photons. Having better values for their mass has helped physicists make estimates of the most likely mass of the Higgs boson, the final piece in the Standard Model puzzle.
Image: Fermilab

Final Years

Even into its last few years, the Tevatron has still managed to find new particles. This year, it spotted the Xi Sub B baryon, a particle that had been predicted to exist but had never been previously observed.
Though the Tevatron has now ceded the title of the world’s most powerful particle accelerator to LHC, there is still a chance for the instrument to find the most sought-after particle in modern physics: the Higgs boson. The Higgs boson is thought to be the reason why particles have the masses they do.
It is estimated that there are nearly 20 petabytes — 20 million gigabytes — of data remaining to be explored in the Tevatron’s archive. Some of this data may contain the elusive Higgs.
“The standard model has been one of the most important theoretical constructs in 20th-century physics,” said Paul Halpern. While it sometimes feels like nearly everything discoverable has been found, he adds that “there’s still a frontier there.”
Image: Paul Halpern